FISKADORO by Denis Johnson (Knopf: $14.95; 221 pp.)

Science fiction at its best can do, in reverse, what certain kinds of history can do. Casting its intuition forward, instead of backward, it illuminates our life.

The intuitions in Denis Johnson's "Fiskadoro" are luminous and suggestive. Thinking into the remnants of what may lie beyond a nuclear holocaust, the novel replants a marooned bit of humanity as if it were a cutting, and recounts the old traits and the new ones that sprout from it.

The mainland of the United States has been destroyed, and perhaps other parts of the world as well. We do not know, exactly, because "Fiskadoro" keeps us in the hauntingly confined circle of its characters' own knowledge. They are a straggler community in the Florida Keys that survives on the edge of the contaminated regions to the north.

It is a quarantined zone, enforced by the undestroyed societies south of it--notably Cuba, which holds the region's power. The quarantine is temporal as well as geographic. It has been in force since the bombs fell, 60 or 70 years earlier, and it will end before very much longer.

The inhabitants of Twicetown--it is Key West and named after the two dud nuclear bombs that fell there and that serve as a kind of shrine--wait in vague apprehension. Their transitional society is about to be replaced; they have no idea how. From the brief words of a narrator at the beginning, we gather that when Cuba, itself transformed, moves in, it will impose what has become an extensive plantation economy and a theocratic regime based on the Koran.

Johnson's novel, beautifully written, does not deal with this future except as a barely suggested shadow. Its focus is on the transitional settlement of fishermen and traders that lives among artifacts of the past, scraps of memories and the uncertain shape of what is to come.

They are a mixture of blacks, whites and Latinos, and they speak an unstable patois with a strong infusion of Spanish. They live by fishing, a kind of scrappy gardening and a few rudimentary crafts bartered with the traders who scavenge the destroyed settlements to the north for bits and pieces of what remains of America's consumer output. They have old records and tapes, amplifying equipment and carefully mended strobe lights for communal celebration. Their huts are furnished with old church pews, car seats, bits of chrome.

Deformed mutants--Los Desechados (the Destitute), they are called--wander through. In the swamps, there is a community of drug-taking primitives whose totem is a two-headed snake--another mutation--and who mutilate their genitals to imitate it.

The three principal characters represent three stages of time. The one at mid-point is Anthony Cheung, a middle-aged clarinetist and vegetable gardener. He does not remember the pre-holocaust civilization but is dedicated to trying to decipher and hold on to bits of it. His parents had their children memorize the Constitution and the Declaration of Independence before burning their copies to keep warm; and in moments of distress, he recites passages from them.

Cheung is the manager of a tiny group of would-be musicians who call themselves the Miami Symphony Orchestra. He is also a member of a circle that gets together to read aloud the few books that remain, including a children's story about whales and Hemingway's "The Sun Also Rises."

He lives with his grandmother, a woman in her 90s who was a Vietnamese refugee. She alone remembers the pre-war world, which for her was a series of flights. In a succession of brilliantly written passages, she recalls bits of her childhood and adolescence. But she can no longer speak, and so the testimony about pre-war civilization that Cheung so covets--the irony is searing--is denied to him.

Cheung is also the mentor of Fiskadoro, a fisherman's son to whom he tries to teach the clarinet. Fiskadoro, whose name is a corruption of an old Spanish word for harpoonist, possesses a kind of prophetic restlessness. He is a misfit among the fishermen and an outsider at the nightly bonfire-lighted beach parties held by his companions. He wanders off across the dunes and is captured by the swamp people. They give him drugs that purge his memory, and they mutilate him.

He returns, finally, and as he convalesces, he gradually re-learns what he has forgotten. But it is knowledge, not memory. Cheung teaches him the clarinet once more, and this time he plays it marvelously well. He has "forgotten how not to play," and it is in this state, unmarked by history, that he becomes a sign of the featureless future, just as Cheung's grandmother is a sign of the past.

Cheung himself, to whom both past and future are shadows, is the immensely appealing balance-wheel of "Fiskadoro." He is an ardent seeker, a frail burning-glass in the fog of history.

Johnson is the author of "Angels," a taut and penetrating novel about a floating American underclass. "Fiskadoro" is a leap of imagination, with no loss of precision and perceptiveness. The book's philosophic explorations are not always clear, but the ambiguities are those of a stunningly delivered poetic vision. Johnson has lassoed a fantastic and imagined future by populating it with people whom we see clearly, and ache for immeasurably, as for ourselves.